1. Field of the Invention
The present invention relates to wires having a low modulus of elasticity. More particularly, the present invention is related to improvements in radiopaque wires; i.e., wires which may be seen under fluoroscopy (X-ray), and methods of making the radiopaque wires. The present invention has particular application to the medical guidewire arts of some of the parent applications hereof, and to the medical stent arts, although it is not limited to the medical arts.
2. State of the Art
Wire is manufactured from ingots using a rolling mill and a drawing bench. The preliminary treatment of the material to be manufactured into wire is done in the rolling mill where white hot billets (square section ingots) are rolled to round wire rod. The action of atmospheric oxygen causes a coating of mill scale to form on the hot surface of the rod and must be removed. This descaling can be done by various mechanical methods (e.g., shot-blasting) or by pickling, i.e., immersion of the wire rod in a bath of dilute sulfuric or hydrochloric acid or mixtures with hydrofluoric acid. After pickling, the wire rod may additionally undergo a jolting treatment which dislodges the scale loosened by the acid. The remaining acid is removed by immersion of the wire rod in lime water.
The actual process of forming the wire is called drawing and is carried out on the metal in a cold state with a drawing bench. Prior art FIG. 1 shows a simple drawing bench 10. The wire 12 is pulled through a draw plate 14 which is provided with a number of holes, e.g. 16, (dies) of various diameters. These dies have holes which taper from the diameter of the wire 12 that enters the die to the smaller diameter of the wire 12' that emerges from the die. The thick wire rod 12 is coiled on a vertical spool 18 called a swift and is pulled through the die by a rotating drum 20 mounted on a vertical shaft 22 which is driven by bevel gearing 24. The drum can be disconnected from the drive by means of a clutch 26. To pass a wire through a die, the end of the wire is sharpened to a point and threaded through the die. It is seized by a gripping device and rapidly pulled through the die. This is assisted by lubrication of the wire. Each passage through a die reduces the diameter of the wire by a certain amount. By successively passing the wire through dies of smaller and smaller diameter, thinner and thinner wire is obtained. The dies used in the modern wire industry are precision-made tools, usually made of tungsten carbide for larger sizes or diamond for smaller sizes. The die design and fabrication is relatively complex and dies may be made of a variety of materials including single crystal natural or synthetic diamond, polycrystalline diamond or a mix of tungsten and cobalt powder mixed together and cold pressed into the carbide nib shape.
A cross section of die 16 is shown in prior art FIG. 2. Generally, the dies used for drawing wire have an outer steel casing 30 and an inner nib 32 which, as mentioned above, may be made of carbide or diamond or the like. The die has a large diameter entrance 34, known as the bell, which is shaped so that wire entering the die will draw lubricant with it. The shape of the bell causes the hydrostatic pressure to increase and promotes the flow of lubricant into the die. The region 36 of the die where the actual reduction in diameter occurs is called the approach angle. In the design of dies, the approach angle is an important parameter. The region 38 following the approach angle is called the bearing region. The bearing region does not cause diametric reduction, but does produce a frictional drag on the wire. The chief function of the bearing region 38 is to permit the conical approach surface 36 to be refinished (to remove surface damage due to die wear) without changing the die exit. The last region 40 of the die is called the back relief. The back relief allows the metal wire to expand slightly as the wire leaves the die. It also minimizes the possibility of abrasion taking place if the drawing stops or if the die is out of alignment with the path of the wire.
Although wire drawing appears to be a simple metalworking process, those skilled in the art will appreciate that many different parameters affect the physical quality of the drawn wire. Among these parameters, draw stress and flow stress play an important role. If these parameters are not carefully considered, the drawn wire may have reduced tensile strength. A discussion of the practical aspects of wire drawing can be found in Wright, Roger N., "Mechanical Analysis and Die Design", Wire Journal, October 1979, the complete disclosure of which is hereby incorporated by reference herein.
The wire forming processes described above may be used to form different kinds of wires. Generally, various characteristics of the formed wire are of interest, depending upon the art in which the wire is to be used. These aspects include, but are not limited to the electrical resistance, tensile strength, and flexibility of the wire. Wire flexibility is particularly important in the medical arts which utilize wires in inter alia stents and guidewires, although wire flexibility is important in other arts as well. For that reason, the medical arts have had much interest recently in nickel-titanium (Nitinol) alloy wires which exhibit superelastic characteristics.
While Nitinol guidewires and stents exhibit desirable wire flexibility for medical procedures, very fine (i.e., small diameter) Nitinol wires are not easily seen during a fluoroscopy procedure. In particular, in the case of guidewires where it is desirable to provide a very flexible distal portion (tip) to the guidewire, the tip of the Nitinol guidewire is typically ground down to a diameter on the order of 0.008" or smaller. With such a small diameter, the X-ray absorption is difficult to detect. Thus, the art has gone to great lengths and expense to provide flexible radiopaque coils around the distal end of the Nitinol guidewire (see, e.g., U.S. Pat. No. 5,520,194 to Miyata et al.). While metal coils formed from material such as platinum operate effectively in providing radiopacity while maintaining flexibility, the coils are difficult to manufacture and attach to the Nitinol guidewire owing, in part, to their dissimilarity with the guidewire material.